Radon gas is radioactive, and it seeps out of the ground all over the world (more in some places than others), collecting in basements and causing lung cancer. It is generated by the radioactive decay of thorium, of which there is a lot in the earth's crust. You can install special fans and venting systems to remove the radon from your house, which may be a good idea if the levels in your geographical area are unusually high.

There was a brief period of time in the early twentieth century during which people thought radon was a good thing, and actually bought products to introduce it into their drinking water. I have an example listed under uranium. (They were wrong.)

Radon generating thorium oxide.
The longest-lived isotope of radon has a half-life of only 3.8 days, which means you can't really collect and store a sample of radon: It would be completely gone within a few weeks. But you can seal up some thorium oxide in a glass tube and be sure you always have some. Radon is one of the decay products of thorium, and there will always be some radon gas trapped in the tube. The concentration should be fairly constant, because thorium has a very long half-life while radon has a very short one: There should by now exist in this tube an equilibrium concentration of radon, which perhaps some helpful reader will calculate for me (necessary information: ca. 0.5g of thorium oxide in ca. 0.5cc volume tube).

The source of this tube, Gillian Pearce, reports that thorium oxide is a better source of radon than is pure thorium metal (which I have much more of), because the radon can't escape from the metal and remains trapped there as it decays.

Mike Seifert, a graduate student in Physics at the University of Chicago, wrote to me with a calculation of how much radon he thinks is in my tube:

Since you asked on your radon page: by my calculations, you have about 5.2 * 10^-17 grams of radon in your tube of thorium oxide. The source of this number is essentially (after a moderate amount of math):

Since the radon-220 lifetime is about 56 seconds while that of thorium-232 is about 1.4 billion years, there really ain't much radon in there... Another way to think about it is that the partial pressure of the radon in the tube is about 1.2 * 10^-6 Pascals, or about 9 * 10^-9 torr. Still better than outer space by about two or three orders of magnitude, though. Ah well, it's the thought that counts. :-)

Sample from the RGB Set.
The Red Green and Blue company in England sells a very nice element collection in several versions. Max Whitby, the director of the company, very kindly donated a complete set to the periodic table table.

Sample from the Everest Set.
Up until the early 1990's a company in Russia sold a periodic table collection with element samples. At some point their American distributor sold off the remaining stock to a man who is now selling them on eBay. The samples (except gases) weigh about 0.25 grams each, and the whole set comes in a very nice wooden box with a printed periodic table in the lid.

Radioactive elements like this one are represented in this particular set by a non-radioactive dummy powder, which doesn't look anything like the real element. (In this case a sample of the pure element isn't really practical anyway.)

Granite sphere.
Radon is a hard element to pin down: You can't see it because it's a clear gas, and you can't even have a vial of it because it's extremely radioactive and has a half life of only 3.2 days. One place it can be found, if only in microscopic and invisible quantities, is in granite rocks. Granite contains uranium and thorium impurities, which constantly give off small amounts of radon gas. This is why areas of the country with certain kinds of bedrock also have problems with radon collecting in basements. And it's why large granite buildings are measurably more radioactive than normal.

I chose this sample to represent its element in my Photographic Periodic Table Poster. The sample photograph includes text exactly as it appears in the poster, which you are encouraged to buy a copy of.

Single-use radon gas detector.
Small vial you put in your basement for a couple of days, then mail in to a lab for testing, to see how much radon you've got.Source:Hardware StoreContributor:Theodore GrayAcquired:29 April, 2009Text Updated:30 April, 2009Price: $10Size: 2"Purity: 0%

Electronic radon gas detector.
Continuous electronic radon gas monitor that reads out in Pico-curies per liter. You can choose between averaging over a few days, or over the whole time the monitor has been installed.Source: CatalogContributor:Theodore GrayAcquired:29 April, 2009Text Updated:28 June, 2009Price: $120Size: 5"Purity: 0%